+UC-SDRL-RJACN-20-263-663/664 Revision: June 12, 2001 +5. FREQUENCY RESPONSE FUNCTION MEASUREMENTS5.1 IntroductionFor current approaches to experimental modal analysis, the frequency response function is themost important measurement to be made.When estimating frequency response functions, ameasurement model is needed that will allow the frequency response functionto be estimatedfrom measured input and output data in the presence of noise (errors).Some of the errors are:•Digital Signal Processing Errors (Leakage, Aliasing)•Noise•Equipment problem (Power supply noise)•Cabling problems (RFI,EMI)•Rattles, cable motion•Calibration (operator error)•Complete system calibration•Transducer calibrationSince the frequency response function can be expressed in terms of system properties of mass,stiffness, and damping, it is reasonable to conclude that in most realistic structures, the frequencyresponse functions are considered to be constants just likemass, stiffness, and damping.Thisconcept means that when formulating the frequency response function usingH1,H2, orHvalgorithms, the estimate of frequency response is intrinsically unique, as long as the system islinear and the noise can be minimized or eliminated.The estimate of frequency response is validwhether the input is stationary, non-stationary, or deterministic. Therefore, several importantpoints to remember before estimating frequency response functions are:•The system (with the boundary conditions for that test) determines the frequency responsefunctions for the given input/output locations.•It is important to eliminate or at least minimize all errors (aliasing, leakage, noise,calibration, etc.) when collecting data.(5-1)

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+UC-SDRL-RJACN-20-263-663/664 Revision: June 12, 2001 +•If all noise terms are identically zero, the assumption concerning the source/location of thenoise does not matter (H1=H2=Hv=Hs=H). Therefore, concentrate on eliminatingthe source of the noise.•Since modal parameters are computed from estimated frequency response functions, themodal parameters are only as accurate as the estimated frequency response functions.There are at least four different testing configurations that can be compared.These differenttesting conditions are largely a function of the number of acquisition channels or excitationsources that are available to the test engineer.In general, the best testing situation is the multipleinput/multiple output configuration (MIMO) since the data is collected in the shortest possibletime with the fewest changes in the test conditions.•Single input/single output. (SISO)•Only option if 2 channel data acquisition system.•Longest testing time.Roving inputs.Roving outputs.•Time invariance problems between measurements.

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